1. Field of the Invention
The present invention relates to sound reproduction systems. More particularly, the field of the invention is speaker enclosures for such audio reproduction systems.
2. Description of the Related Art
Sound reproduction systems are well known, and typically include a generation circuit (e.g., a radio receiver or a compact disc player), an amplification circuit (for amplifying the signal from, e.g., the radio or disc player), and a speaker consisting of one, two, or three, etc., transducers; a typical three-way speaker system consisting of high frequency transducers (tweeter, HF), a mid-range transducer (middle range frequencies, MF), and a low frequency transducer (woofer, LF). Various arrangements are known for speaker designs for each of these types of transducers, and the speaker design may be specially constructed to benefit the audio band which the transducer produces. In particular, low range transducers (woofers) are well known to include a cone diaphragm (typically 4xe2x80x3 to 30xe2x80x3) which vibrates according to the signal received by the transducer. The diaphragm may be conically shaped, and while air on one side of the diaphragm is compressed, air on the opposite side of the diaphragm is rarified. This results in the sound resonating from one side of the transducer being 180xc2x0 out of phase with sound resonating from the other side of the transducer. In order to separate the compressed air from the rarefied air, a low frequency transducer must be placed in an enclosure designed to isolate its front radiations from rear radiations. There are many such configurations.
One of the simplest is a circular baffle with the transducer mounted in the center. This configuration requires a baffle approximately 40 feet in diameter to produce bottom A on a piano or 27.5 Hz.
A large sealed enclosure with the transducer mounted in such a manner as to isolate the transducer front radiation from its rear radiation is known as an infinite baffle. Large infinite baffles produce good low range sound but are relatively large and only utilized one side of the mounted low frequency transducer.
An infinite baffle cabinet can be reduced in size by adding a vent or port. This configuration is known as a reflex cabinet or vented box and produces low frequency more efficiently and with better impedance match to an amplifier than the infinite baffle but does not produce low frequency sound as xe2x80x9copenxe2x80x9d as the above baffle.
To achieve a more open sound and a good impedance match to an amplifier, a cabinet configuration referred to as an acoustic labyrinth can be employed.
An acoustic labyrinth is a pipe or duct, folded to conserve space, and connected to the rear of the transducer cone. The far end of the duct is left open and in the same plane as the transducer. The pipe is arranged to be a quarter wavelength at some low frequency near to the resonant frequency of the loudspeaker when loaded by the labyrinth. Around this frequency, the transmission delay along the pipe brings the energy radiated from the open end approximately in phase with the front radiation on the unit. The low frequency range of the system is therefore reinforced, and since the transducer is looking into a high impedance when the pipe is a quarter wavelength, cone excursions are much reduced, resulting in less non-linear distortion. A loudspeaker with a loaded resonance at 40 Hz would require a duct 7xe2x80x2 long. At frequencies below the quarter wavelength mode, the port and diaphragm radiation are out of phase and the response falls off steeply. At higher frequencies, the port radiation would be in and out of phase according to wavelength, and would tend to produce irregular response characteristics in the middle registers; sound absorption is therefore necessary.
The labyrinth actually lowers the frequency of cone resonance owing to the mass of air in the duct which operates directly on the cone surface. As enclosures go, it produces very good bass. G. A. Briggs, Loudspeakers, Wharfedale Wireless Works Limited, 1948, Chap. 18, xe2x80x9cCabinetsxe2x80x9d, p. 191.
The transmission line (TL) has its design roots in the Stromberg-Carlson acoustic labyrinth (circa 1930). It first consisted of a long pipe (open at one end and the driver mounted at the other), with a cross-sectional area about the same as that of the driver. The line length was made about 25% of the driver resonance""s wavelength and then folded to make it into a practical shape. Without any stuffing or damping material in the line, the enclosure dampened output at resonance, and reinforced the frequencies about one octave above resonance.
Working with the same basic concept in the early 1960""s, A. R. Bailey experimented with different damping materials and techniques in folded labyrinth lines. A. R. Bailey, xe2x80x9cA Non-Resonant Loudspeaker Enclosure Designxe2x80x9d,Wireless World, October 1965; T. Jastak, xe2x80x9cA Transmission Line Speakerxe2x80x9d, Audio Amateur, January 1973;. A. R. Bailey, xe2x80x9cThe Transmission Line Loudspeaker Enclosurexe2x80x9d, Wireless World, May 1972. This work has since become the basic bible for most TL designs. Using Bailey""s density criteria of 0.5 lb. cu. ft., A. J. Bradbury published his 1976 paper (A. J. Bradbury, xe2x80x9cThe Use of Fibrous Materials in Loudspeaker Enclosuresxe2x80x9d, JAES, April 1976) which described changes in the speed of sound for different types of damping material (fiberglass and long fiber wool). Vance Dickason, The Loud Speaker Design Cookbook, Audio Amateur Press, Peterborough, N.H., 1995, Chap. 4, xe2x80x9cTransmission Line Low-Frequency Systemsxe2x80x9d, p. 73.
Transmission lines are enclosures filled with a sound absorbing material, such as long fiber Dacron wool, in order to delay the low frequency acoustic waves so that the transducer compression and rarefactions are in phase, i.e., that cancellation does not occur. Packing the enclosure with sound delaying and absorbing material (equal to 0.5 lb. cu. ft. of wool or more) produces an undesirable acoustic result by dramatically reducing the xe2x80x9copennessxe2x80x9d of the bass sound.
The present invention is a speaker system which has an enclosure about one-eighth of the length of the lowest frequency sound which one wants to produce using a transducer.       wavelength    =                  speed        ⁢                  xe2x80x83                ⁢        of        ⁢                  xe2x80x83                ⁢        sound        ⁢                  xe2x80x83                ⁢        in        ⁢                  xe2x80x83                ⁢        air                    desired        ⁢                  xe2x80x83                ⁢        frequency        ⁢                  xe2x80x83                ⁢                  (                      c            /            s                    )                                cabinet      ⁢              xe2x80x83            ⁢      length        =          wavelength      8      
The enclosure defines an interior space having two ends, one end mounting the speaker and the other end having an opening which allows the rearward directed sound waves to escape. The sound produced by the interior side of the speaker travels for one-eighth of a wavelength of the sound to be produced to the opening, and travels an additional one-eighth of a wavelength to the front side of the speaker so that the sound is in phase with the sound waves produced by the exterior face of the transducer. The interior channel of the enclosure of the present invention is open (i.e. not obstructed by excessive sound delaying stuffing) to provide a high quality, life-like xe2x80x9copenxe2x80x9d bass sound. Tweeters and mid-range speakers may also be accommodated within the speaker enclosure by including a layer of sound absorbing material on the interior wall or walls of the enclosure or utilizing fibrous material at such a density (less than 0.5 lb. per cu. ft.) that low frequencies (approximately 150 cps or less) are not absorbed or isolating them in separate small cabinets within the main enclosure. The enclosure may have a cross-sectional shape of circle, triangle, square, rectangle, trapezoid, pentagon, hexagon, heptagon, octagon, other polygons, etc., and may be disposed horizontally or vertically. A conically shaped baffle may be used to distribute the rearwardly transmitted sound out of the speaker enclosure, and is particularly useful with vertically disposed speaker enclosures. Further embodiments of the present invention include providing two or more transducers mounted in oppositely directed chambers. A transducer mounted at one end of an enclosure having internal baffles which create a serpentine path for sound, a transducer mounted in the center of an enclosure and other variations. The present invention provides a sound path from the transducer to the enclosure opening having a distance of approximately one-eighth of the wavelength of the low frequency sound to be produced. The sound escaping through the opening thus will be in phase when it reaches the front of the speaker, thereby utilizing 100% of both sides of the woofer and at the same time creating a full and open bass sound. By taking advantage of phase reinforcement, the efficiency of the speaker is increased because the sound energy from the interiorly facing side of the transducer diaphragm is effectively combined with the sound energy emanating from the exteriorly facing side of the speaker diaphragm. Also, this produces a flatter impedance curve which results in a better transducer match to the amplifier output. The present invention thus has a greater power handling capability because of its increased efficiency and matching with the amplifier output.
Conventional cross-over circuits are utilized to limit the frequency of the sound produced by the bass transducer to a low level, for example below 150 hertz or, preferably, below 75 hertz. If a mid-range transducer and/or tweeter are disposed within the enclosure so that higher frequency sounds are transmitted through the interior channel of the enclosure, means are provided for absorbing these higher frequency standing waves. One such technique is to line the interior wall or walls of the enclosure with sound absorbing material. However, it is critically important that the channel itself remain an open cross-section area and should not be less than approximately 25% of the woofer""s effective cone radiating area so that the low frequency acoustic waves can travel rearwardly through the channel and out the opening without being damped or delayed. This combination of high frequency attenuation and completely unobstructed transmission of the low frequency acoustic waves produces the high quality open bass sound that is characteristic of the present invention. The present invention involves, in one form, a speaker system for the reproduction of low frequency sounds. The speaker system comprises a low frequency transducer for reproducing sounds in a low frequency band, a filter, and an elongated enclosure. The filter provides signals for reproducing sound which does not exceed 150 hertz in frequency, and is connected to the transducer and is adapted for connection to the audio system. The enclosure has an interior channel having two ends. The low frequency transducer is mounted at one end and has a front vibrating surface facing exteriorly and a rear vibrating surface facing interiorly. The other end has an opening adapted to allow sounds produced by the speaker to escape the interior space. The distance between the speaker and the opening is approximately one-eighth the length of the wavelength of the lowest frequency sound to be produced by the speaker.